Perpendicular neutral beam injection into the stellarator W7-AS

The radial electric field E-r is of crucial importance for the global confinement properties of a stellarator plasma. In particular, in the long-mean-free-path regime, which is characterized by strongly enhanced neoclassical transport, the ambipolar E-r can help to reduce the unfavourably large neoclassical transport coefficients. In the stellarator Wendelstein 7-AS (W7-AS), E-r is increased by the almost perpendicular injection of highly energetic neutral particles. The induced fast-ion orbit losses, drive the ambipolar E-r more negative. Changes of the radial electric field up to -20 000 V m(-1) are attained in the gradient region of W7-AS. The observed increase of the global energy confinement time is in the range of 10% in the best examples. In parallel to the experiments, numerical Monte-Carlo simulations of the ion orbit losses and the formation of the nonambipolar E-r are performed.